HUBBLE REVEALS DETAILS OF A NEWLY BORN PLANETARY NEBULA

Astronomers have caught a peek at a rare moment in the final stages of a
star's life: a ballooning shroud of gas cast off by a dying star
flicking on its stellar light bulb. NASA's Hubble Space Telescope has
captured the unveiling of the Stingray nebula (Hen-1357), the youngest
known planetary nebula. Twenty years ago, the nebulous gas entombing the
dying star wasn't hot enough to glow.

The Stingray nebula (Hen-1357) is so named because its shape resembles a
stingray fish. Images of a planetary nebula in its formative years can yield
new insights into the last gasps of ordinary stars like our sun.

A planetary nebula forms after an aging, low-mass star swells to become
a "red giant" and blows off some of its outer layers of material. As the
nebula expands away from the star, the star's remaining core gets hotter
and heats the gas until it glows. A fast wind - material propelled
outward from the hot central star -- compresses the gas and pushes the
gas bubble outward.

The central star in the Stingray nebula has heated up quite fast. "Such
a fast evolution of the object actually came as a surprise," says
Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md.
"The current theoretical models do not predict such fast evolution for
low-mass stars like that of the Stingray nebula."

Adds Matt Bobrowsky of Orbital Sciences Corp. in Greenbelt, Md: "The
Stingray nebula is, in human terms, just an infant because only within
the past 20 years did its central star rapidly heat up enough to make
the nebula start to glow. It is extraordinary to catch a star in this
exceedingly brief phase of its evolution. While stars typically last for
billions of years, the transition to a visible planetary nebula takes
only about 100 years -- the blink of an eye compared to a star's
lifetime. It is therefore not surprising that no younger planetary
nebula has ever been identified."

The nebula is one-tenth the size of most planetary nebulae and is 18,000
light-years away in the direction of the southern constellation Ara (the
Altar). Because of its small size, no details of the Stingray nebula
were visible before Hubble observations were carried out.

The creation of twin bubbles of gas, which shape so many planetary
nebulae, has always been a mystery to astronomers. The jets of gas
revealed in the Hubble images are of great interest to astronomers
because many types of astronomical objects - from young stars to active
galaxies - produce similar, opposing flows of gas. Many theories have
been proposed to explain these jets, but the details of their formation
are not yet fully understood. The Hubble images actually reveal how the
jets in the Stingray are produced.

"Both theory and observations have indicated that a ring or disk of
matter plays a role in forming the opposing outflows," Bobrowsky says.
"But these images are significant in showing that, at least in some
cases, the situation is somewhat more complex."

The images that Bobrowsky and collaborators acquired show a ring of gas
surrounding the central star, with bubbles of gas above and below the
ring. The wind emanating from the central star has created enough
pressure to blow open holes in the ends of the bubbles, allowing gas to
escape. The bubbles act like nozzles that direct the escaping gas into
two opposing streams. The images also show bright gas that is heated by
a "shock" caused when the central star's wind hits the walls of the
bubbles.

A further discovery is a second star within the nebula, indicating that
the Stingray's central star is part of a binary star system. This second
star is important because astronomers have theorized that a companion
might be necessary for the formation of the ring, bubbles, and columns
of gas.

There is also evidence that some of the gas in the nebula may be
distorted due to the gravity from the companion star - another
phenomenon never before seen in a planetary nebula. This appears in the
images as a spur of gas forming a bridge to the companion star.

Bobrowsky first observed the Stingray nebula with the Hubble telescope
in 1993. Those images were the first to show the structure of the
nebula. The new observations were taken in 1997 by Bobrowsky and his
collaborators: Sahu of the Space Telescope Science Institute, M.
Parthasarathy of the Indian Institute of Astrophysics in Bangalore,
India, and Pedro Garcia-Lario of the ISO Science Operations Center in
Madrid, Spain. The results are described in the April 2 issue of the
journal Nature.

The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) for NASA,
under contract with the Goddard Space Flight Center, Greenbelt, MD.
The Hubble Space Telescope is a project of international
cooperation between NASA and the European Space Agency (ESA).

HUBBLE CAPTURES UNVEILING OF PLANETARY NEBULA

This Wide Field and Planetary Camera 2 image captures the infancy of the
Stingray nebula (Hen-1357), the youngest known planetary nebula.

In this image, the bright central star is in the middle of the green
ring of gas. Its companion star is diagonally above it at 10 o'clock. A
spur of gas (green) is forming a faint bridge to the companion star due
to gravitational attraction.

The image also shows a ring of gas (green) surrounding the central star,
with bubbles of gas to the lower left and upper right of the ring. The
wind of material propelled by radiation from the hot central star has
created enough pressure to blow open holes in the ends of the bubbles,
allowing gas to escape.

The red curved lines represent bright gas that is heated by a "shock"
caused when the central star's wind hits the walls of the bubbles.

The nebula is as large as 130 solar systems, but, at its distance of
18,000 light-years, it appears only as big as a dime viewed a mile away.
The Stingray is located in the direction of the southern constellation
Ara (the Altar).

The colors shown are actual colors emitted by nitrogen (red), oxygen
(green), and hydrogen (blue). The filters used were F658N ([N II]),
F502N ([O III]), and F487N (H-beta). The observations were made in
March 1996.